Amplitude modulation radio beacon



Nov. 15, 1966 C. W. EARP AMPLITUDE MODULATION RADIO BEACON Filed Marcha, 1964 5 Sheets-Sheet l if 5( /7f 9F I I I QE WE muMLEA/T PH4SDW/AT/O/V I'nuenfor CHARLES lA/. EARP Nov. 15, 1966 c. w. EARP 3,286,262

AMPLITUDE MODULATION RADIO BEACON Filed March 6, 1964 5 Sheets-Sheet 2SIDEBAND AMPL/ 7110f AMPL/ TUBE AZ/MUTH lnvmlor CHARLES kl. EARP Ahorney Nov. 15, 1966 C. W. EARP AMPLITUDE MODULATION RADIO BEACON FiledMarch 6, 1964 ILLI 3 Sheets-Sheet 5 PARALLEL FEED Inventor CHARLES W-EARP A ttorny United States Fatent O 3,286,262 AMPLITUDE MQDULATIQNRADH} BEACQN Charles Wiliiam Earp, London, England, assignor toInternational Standard Electric Corporation, New York, N.Y., acorporation of Delaware Filed Mar. 6, 1964, Ser. No. 350,040 Claimspriority, application Great Britain, Mar. 13, 1963, 9374/ 63 17 Claims.(Cl. 343-107) This invention relates to radio approach beacons in whichthe output of a transmitter is coupled in turn to a number of aerialsdisposed on a line in order to simulate a radiating source in linearto-and-fro motion.

A craft equipped with a co-operating receiver can navigate with respectto such beacons by evaluating the deviation of a frequency modulationsignal apparently transmitted by the beacon. The frequency modulationarises because the apparent movement of the simulated source results ina varying Doppler frequency shift.

It is often more desirable to navigate a craft according to a receivedamplitude modulation.

The invention in one aspect provides a radio beacon including meanseither to eifect or to simulate the oscillatory movement with mutuallyopposite phases of a pair of radiating aerials, the movement orsimulated movement being such that an amplitude modulated radiated fieldis produced by the two aerials.

According to another aspect of the invention, there is provided a radiobeacon including means to simulate oppositely phased movements along aline of a pair of aerials energized with radio frequency energy inopposite phase to each other by cyclic successive coupling of spacedaerials to a transmitter.

In one preferred embodiment of the invention, the opposite oscillatorymovements are not simple harmonic, but are linear symmetrical saw-toothmovements.

In another preferred embodiment, there are simulated by successivecoupling four such pairs of sources in opposed saw-tooth oscillatorymovements, the phases of the opposed movements being synchronized.

In all of the embodiments of the invention to be described there is alsoradiated continuously from one or more aerials a wave in quadrature withboth of the waves of the pair or each pair of the apparently movingsources for the detection of the amplitude modulation in a cooperatingreceiver. This wave is not successively switched to different aerials.

Embodiments of the invention will now be described with reference to theaccompanying drawings, in which:

FIG. 1 shows diagrammatically a line of aerials and a transmittereffecting their successive energization;

FIG. 2 is a graph showing the relation between the amplitude of varioussidebands for varying azimuth directions from the line of FIG. 1 whenthe aerials are energized successively to simulate a pair of sourcesmoving oppositely according to a linear symmetrical sawtooth law cycle;

FIG. 3 shows similarly the A.M. sideband amplitudes for two pairs ofsimulated sources in opposite linear symmetrical saw-tooth movements;

FIG. 4 shows the sideband amplitudes when four such pairs are simulated;

FIG. 5 shows diagrammatically an electronic switching arrangement forsuccessive aerial coupling; and

FIG. 6 shows the sideband amplitudes set up in one arrangement when twodifferent kinds of source movement are simulated simultaneously.

Referring to FIG. 1, there are shown two aerials 1 at each end of a rowof N equispaced similar aerials disposed on a line AB. Each of the Naerials 1 is coupled by a respective transmission line 2 to a respectivefixed contact 3 of a switching device. The switching device has, inaddition to the N fixed contacts 3, a pair of moving contacts 4 and 5coupled to the output of a VHF transmitter 6. The contact 4 is directlycoupled while the contact 5 is coupled to the transmitter output via a180 phase shifter 7.

All the transmission lines 2 introduce equal or negligible phase shifts.The aerial spacing is slightly less than 7\/2 at the frequency of thetransmitter 6 so that all-round guidance may be obtained. If guidancewere not required at an angle greater than 0 from the perpendicularbisector of the array, the aerial spacing should not exceed A/ 2 cosec0.

The moving contacts 4 and 5 are driven by means not shown so as to sweepalways in opposite directions over all the fixed contacts 3 cyclicallyfrom end to end.

This arrangement enables the simulation of a pair of sources radiatingwaves of equal amplitude, but oppositely phased and moving always inopposite directions to and fro along the line AB, one source thereforeapparently moving towards B whenever the other moves towards A.

The output of the transmitter 6 is coupled also to a central one of theaerials 1 via a phase shifter 8 so that from this central aerial is afixed source of radiated waves in quadrature with those from thesimulated moving sources.

The three radiated waves produce a radiation field around the line ABwhich is an amplitude modulation, there being many sidebands present,even though each moving source separately creates a frequency modulationfield pattern due to the Doppler eifect.

The nature of the A.M. field depends on the length of the line AB, andalso of the law of motion of the simulated sources in describing theline AB. This law of motion will depend among other things on thespacing of the contacts 3 of the switching device.

In one embodiment of the invention, the spacing of the fixed contacts 3is such that reciprocation of the moving contacts 4 and 5 provides asimple harmonic oscillatory motion of the two oppositely movingsimulated sources.

The resulting field is a set of odd-order sidebands of the frequency ofthe energization cycle. At all azimuths, the even-order sidebands andcarrier wave have cancelled, but all of the odd-order sidebands arepresent. Apart from very near-field efiects, the field is a pureamplitude modulation, and one sideband is filtered in a co-operatingreceiver so that the equipped craft can navigate.

In another embodiment of the invention, fixed contacts 3 in FIG. 1 areequispaced and the moving contacts 4 and 5 move over them to and fro atconstant velocity. Thus the simulated moving sources each describe theline AB at constant velocity before reversing in direction. In otherwords each source apparently describes a linear symmetrical saw-toothmovement.

The radiation field from this embodiment has the advantage over thatjust described that the odd-order amplitude modulation sidebands areseparated in azimuth as is described more fully below with reference toFIG. 2, so that the problem of separating overlapping sidebands is lessacute.

FIG. 2 shows the nature of the A.M. field set up around the energizedaerials for apparent linear symmetrical saw-tooth motions. The abscissais the equivalent phase deviation, due to the simulated movement, ofwaves from one source and is equal to 1r.AB sin 0, Where 0 is the anglemade by the bearing line from the point in space observed with theperpendicular bisector of the line AB. Since the line AB is nearly N \/2long, an equivalent phase deviation of 1r will correspond to an angle 03 of about sin- (2/N). The maximum phase deviation equivalent will beobserved from in line with the array and is equal to 1r/X.AB.

The curve marked F is the magnitude of the sidepairs of oppositelymoving simulated sources, this time spaced irregularly in time-phase ofmovement at 36, 60 and 96. The movements are again linear syrnmetricalsaw-tooth in form and zero output this time ocband at the frequency F ofthe energization cycle, i.e. the 5 curs at 3F, 5F and 9F as shown inFIG. 4. Thus little frequency at which a simulated moving sourcedescribes transmitter power is wasted in setting up radiation fields Ato B and back to A. The curves 3F, 5F, 7F and 9F at these frequencies,while near to the course-line the F are the magnitudes of the sidebandsat these frequencies. sideband is boosted to 3.3 times the voltage whichwould It will be observed that the various sidebands each be radiated bya single pair of oppositely moving sources maximize at a differentequivalent phase deviation, for 10 excited in opposite phase. instance,the curve F has a maximum at 1r/ 2. At devia- The above arrangementshave all been described with tions other than 1r/2, 31r/2, 51r/2, etc.,more than one reference to FIG. 1 which shows a mechanicalswitchsideband is present, thus the signal envelope is not a pure ingarrangement. It is immaterial what mode of switchsinusoid. ing is usedso long as the pair or pairs of oppositely mov- The pattern shown inFIG. 2 is that on one side of ing sources as described are simulatedsufficiently well for the perpendicular bisector of AB only, on theother side a distant receiver to receive signals which do apparently thepattern is the same but the actual A.M. has the opcome from actualmoving sources. In these arrangeposite phase. ments the aerials shouldnot be more than \/2 apart.

On the horizontal perpendicular bisector, the A.M. Since the carrieraerial which is fed in quadrature magnitude is zero, and thus a receiveron this line will with the switched waves is centrallydisposed, and thenot detect any A.M. In order that this condition may sources of eachpair are oppositely moving, they are albe differentiated from -abreakdown, an amplitude modways symmetrically disposed with reference tothe carrier ulation may be applied to the wave which is radiated fromaerial, and so the latter will only suffer equal and opa fixed point,the deviation of the receiver from this line posite excitations due toany aerial interaction effect. will add to or subtract from thisinitially radiated A.M. 5 This quadrature wave may if desired be sentfrom a according to the sense of the deviation. separate central aerial.

In a further embodiment the switch shown in FIG. 1 When an aerial isscheduled to be excited by waves of has added to it a further pair ofoppositely moving conopposite phases at once, it is better not to exciteit at all. tacts (not shown) moving over the fixed contacts 3 with Amore detailed description of an approach beacon the same frequency F butdiffering in time phase from 3() having equipment for feeding fouraerials in a line to the movement of the contacts 4 and 5 by 60.simulate two pairs of oppositely moving sources with a The result isthat there are simulated a second pair of 60 time phase differencebetween the simulated IIHOVC- oppositely moving radiating sources overthe same track ments will now be described with reference to FIG. 5. butdelayed by 1/6F seconds from the movements of the Four aerials A A A andA spaced just less than first pair. M 2 to a line, have means toenergize them in turn at equal Due to these four simulated radiators,the F sideband intervals by connection to two antiphased outputs of awill now be the resultant of two vectors inclined at 60, transmitter inrepeated cycles according to the following while the 3F sideband willresult from 180 spaced vectors. schedule:

Coupling step 1 2 3 4 5 6 Aerials energized in one phase A1 and A2"- A:and As--. A; and A4. A4 and A3". A; and A}--. A, and A Aerials energizedin opposite phase A4 and A3.-- A; and As.-. A; and A1.-. A1 and A}... A;and 11;.-- A; and A4.

Thus compared with FIG. 2, the F sideband is boosted by At each step twoaerials are connected to each of the 1.73 and the SF sideband is zero.This provides even betoppositely phased tnansmitter outputs, so that twopairs ter space isolation of the F sideband from other sidebands. ofoppositely moving sources are simulated. One pair The 9F sideband willalso be zero. The SF and higher of sources moves one step of 60 behindthe other, so sidebands may be removed, as will be described later, bythat the resulting radiation field resembles the spectrum having theline AB of the array so short that these higher shown in FIG. 2 with the3F sideband removed, and the F sidebands have not a significantamplitude anywhere sideband boosted by 2 sin 60. around the beacon.There is one difference, however, in that the maximum In extension ofthe same principle, a further two pairs equivalent phase deviationpossible is 31r/2 because the of oppositely moving antiphased sourcessimulated by length of the aerial array is only approximately 31r/2, sointroducing further moving contacts or further switching that the SF and7F sidebands are negligible. devices can be made to cause even morereinforcement of The embodiment shown in FIG. 5 will only radiateselected bands at the expense of others, and hence better at asignificant amplitude the first pair of sidebands over use of availabletransmitter energy. If to the two th h l range f i th Spaced Pairs of ppy moving Sources mentimled 60 Referring in more detail to FIG. 5, theapparatus inabOVe, there are added further two Pairs of Spaced cludes2700 c./s. oscillator 10 feeding a X5 divider 11 pairs, such that thereare four pairs each separat d from which feeds a ring-of-six divider 12.The latter has six the next one in time phase by :30 This involves theseparate Outputs X1, X2 X3, X4 X5 and X6 which each ilmulanori of eightseparate movingsources {our Iadlat' carry pulses in turn at ninety persecond in response to the ing a carrier of the one phase, their timephases of to-and- 540 c 1 o a o yc es sec. input. fro movement bemg(say) 0 30 60 and 90 and four Th 1 edt n t 13 b more radiating a carrierof the opposite phase, tirneepu us e i means phased in movement at 1802100: 2400 and 2700 (Le. of shunt switching diodes in four bridgecircuits X so moving in opposite to respective ones of the first four).that the respgctwe aenals and A4 are fed only FIG. 3 shows the spectrumof the radiation field from when denfandefl the 'f 5hedu1ethisarrangement, which also boosts the F sideband while The clrcmts XCompnse three hues of M 4 P cancening h 3 sideband, h SF and 71::sidebands length and one 3)\/ 4 line connected end to end in a square.are much weaker than the F sideband while that at 9F At two oppositecomers the connecticns are Via Shunt is also zero. switching diodes11,06. The other two connected corners of In yet another arrangementthere are also four separate 75 each bridge circuit X are coupled to theaerial A and to the transmitter output. A pulse x x or x applied toeither diode a, a makes the diode conduct, and the phase of the thusungated transmitter signal reaching the aerial A is in one direction orin that direction directly opposite according to whether diode a or a istriggered, due to the presence of one 3M4 line in the bridge X.

For instance, referring to the first step :of the above schedule, thepulse x is applied to the diodes a a a and a;, as the first couplingstep, and the next but one phase x;; to diodes a a a' and a It will beseen that the pulses x and x are scheduled to switch both diodes of twobridges, which would short-circuit the transmitter out-put. To avoidthis, the transmitter is either immobilized or made to feed a matchedload. This does not disturb the radiation field since the feeding of anaerial A is two opposite phases in equivalent to no feed at all.

Thus one pair of simulated sources lags the other by 60 (one couplingstep at six to the cycle) and the field shown in FIG. 3 is set up, Fbeing equal to 90 c./s.

In this embodiment the two centre aerials A and A are also excited withthe transmitter output in quadrature with the four simulated sources andthis continuous enengization carries an amplitude modulation derivedfrom the oscillator at 90 c./s., so that this modulation adds to thatproduced by the simulated source movement on one side of the horizontalperpendicular thnough the array, and substracts from it on the otherside.

Simultaneously with the two pairs of simulated sources oppositely movingat 90 c./s. lt-o-and-fro on the aerial line, there are simulated bymeans of the same RF transmitter two more pairs of sources moving in anexactly similar manner at 150 c./s., so that there is superimposedanother similar radiation field, this time F being 150 0/5.

An amplitude modulation also derived from the 2700 0/5. oscillator 10 at150 c./s. is superimposed on that at 90 c./s. on the carriercontinuously fed to the central aerials A and A but is phased so thatthis 150 c./s. modulation adds to that due to the moving sources on theother side of the perpendicular through the array and subtracts on thefirst side; i.e. contrary to the 90 c./s. modulation.

Thus a craft equipped with a cooperating receiver will detect equalsignals when on the perpendicular bisector which is the ordinate axis inFIG. 2. The 90 c./s. A.M. will predominate on one side of the bisectorand on the other side the 150 c./s. A.M. will predominate. Hence thecraft can tell the sense of its displacement from the horizontalperpendicular bisector through the aerial array.

Returning to the equipment shown in FIG. 5, the signal from theoscillator 10 is divided by three in a divider 14 and the resulting 900c./s. signal further divided by six in a second ring-of-six divider 15which, like divider 12, provides output pulses on six separate leads Y Yin turn this time at 150 c./s. This also provides a 150 c./s. output forthe modulation of the quadrature output of transmitter 13 in a modulator16, which also receives a 90 c./s. signal from the divider 12. Thepulses y y are fed to switching diodes on the Y bridges to feed theaerials A A A A; as did the pulses from x x In order that each aerialmay be fed via a bridge X or a bridge Y or by both at the same timewithout detri ment to the transmitter, each of the bridges X and Y feedinto opposite corners of a further bridge Z which also has three 4 linesand a 3M 4 line. The aerial is coupled to a third corner of the bridgeZ. By this means negligible coupling exists between a bridge X and abridge Y.

It will be understood that this electronic method of successive aerialswitching may be replaced by any other method so long as the scheduleillustrated above is carried out.

The number of aerials fed continuously with the wave in phase quadraturewith the oommutated Waves is immaterial except that the feeding of morethan one results in a directive effect.

The method of indicating the sense of the deviation of a craft from theline of symmetry by using two commutated frequencies is not essential tothe invention. The extra 150 c./s. apparatus designated Y in the diagrammay be omitted if sense of deviation is not required, or may be replacedby other sense indicating means.

Another example of the invention will now be described in which thereare twelve aerials A A positioned equispaced on a line of total length4%.

In this example, the linear symmetrical saw-tooth switching cycle ismaintained, but the commutation is performed such that four pairs ofoppositely moving antiphased carrier sources are simulated. The generalmethod of switching is similar to that illustrated for the lastembodiment, except that the end aerials A and A are energized always fortwo coupling steps to avoid having to energize any aerial twice during asingle time element via the same X bridge.

The four pairs of oppositely moving sources are spaced in movement at 30intervals in order to produce the radiation field spectrum shown in FIG.3. Since the end aerials are coupled each time for two consecutivecoupling steps, there are twenty-four steps and so a 30 interval betweensources corresponds to two steps.

The coupling cycle to one phase of the transmitter will thus be insuccessive coupling steps:

and to the other phase the corresponding steps will be:

12 11 2 1 1 2 11 12 12 11 A10A9 A3A4 A11A10A9 A A A A A A A A A At anycoupling step eight aerials are coupled via an X bridge to simulate thec./s. movements, and eight are coupled via a Y bridge to simulateidentical movements at c./s.

The apparatus in this'embodiment is analogous to that shown in FIG. 5,the main diiierence being that since more pulses are required in the Xand Y coupling cycles, the oscillator 10 is tuned to 10,800 c./s. whichis divided as before by 5 and 3 for the X and Y circuits, and ringof-24dividers are used instead of ring-of-six dividers.

Peak output at the frequency F corresponds to sinl/ 8 or 7 from theperpendicular bisector course line.

The signal in quadrature with the waves from the simulated sources isagain modulated at 150 c./s. and 90 c./s. so that the sense of deviationfrom the course line can be determined, and is again coupledcontinuously to the two centre aerials (A and A The radiation field isgenerally as shown in FIG. 3 for both F=90 and F=l50 c./s. The amplitudemodulation due to the non-commutated signal is added or subtracted tothat shown according to which side of the perpendicular bisector of thearray (i.e., the ordinate axis in FIG. 3) the co-operating receiver islocated and in opposite senses according to whether the F =90 c./ s. or150 c./s. modulations are being considered.

One further embodiment and a variation thereof will now be describedwhich differs from the last embodiment only in that an additionalcoupling sequence is added to all the others so that a sideband atfrequency F appears to a small extent for all azimuth bearings aroundthe beacon.

Referring to FIG. 3 which showed the nature of the radiation field setup by the radiations of the last embodiment, it will be seen that theamplitude at the first sideband frequency F is insignificant forequivalent phase deviations above 31r/2, i.e., i22 azimuth. In thisembodiment another radiation field is superimposed on that shown in FIG.3 so that in the combined field, the total 7 amplitude modulationsideband at frequency F does not equal zero anywhere, although itsamplitude is smaller beyond 311'/ 2.

Additional equipment is provided to energize the middle All of theembodiments require the co-operating receiv ers to be equipped only withamplitude modulation detection equipment, and the drawback of morecomplicated switching systems in some of the embodiments is compensixaerials A A from the transmitter at a cyclic sated by having more of thetransmitter power concenfrequency of 30 cycles per second in order tosimulate trated in low order sideband. three further pairs of oppositelymoving antiphased Longer aerial arrays give a Wider base for thetranssources. The three further pairs are phased at equal mission, andhence a more interference-free field, but may (120) intervals so thatthe fundamental amplitude moduresult in more sidebands being present.lation sideband at 30 c./s. in the resulting field cancels to 0 Oneadvantage of the systems described herein is that zero, as do the fifthand seventh order sidebands. The for an aerial array of given length,the sharpness of the third order A.M. sideband at 90 c./s. is boosted tothree course defined by the beacon is twice that of I.L.S. beatimes thenormal value and this lobe extends from 1r/ 2 to cons now in use. Siteerrors are probably reduced by a 51r/ 2 equivalent phase deviation.similar factor.

Since only half the aerial array is now being commu- Another advantageis that of compatibility with I.L.S. tated at 30 c./s., the abovedeviations correspond to from approach beacons approved by I.C.A.O. 1rto 571 on the field shown in FIG. 3. As stated previously, to anobserver situated at an angle FIG. 6 shows the total field at 90 nowobtained by 6 from the perpendicular through the array, the desired thesuperimposition of the two radiation fields. In this movements will notappear to be correctly simulated if figure, 90 c./s. amplitude isplotted against both equivvthe aerial Spacing exceeds A eosee Thus ifthe p alent phase deviation for a single source and against azig LeXCedS the beacon Cannot be relied on for ng muth calculated from the 4%length of the array. greater than Sin-1 U- This is the only embodimentdescribed herein in which It is to e und rst d that th f r g ingdescription a third order sideband has been utilized, but those skilledof specific examples of this invention is not to be conin the art canclearly extend the principles of simulated sidered as a limitation ofits scope. oppositely moving antiphased sources by progressive aerialWhat I claim is coupling to utilize the higher order sidebands. A radiobeacon for Predlleing amplitude modu- The three twin commutation cyclesat may be lated field, comprising an array of spaced aerials, atransperformed instead of or as well as at 50 c./s. to reinforce mitterhaving first and Second outputs of pp P the 150 c./s. fieldcorresponding to FIG. 3 in the same 30 means for coupling said firstoutput of said transmitter way. successively and cyclically to each ofsaid aerials in said There aerial couplings are actually preformed usinga array, and means for coupling said second output of said ring-of-IZdivider and another set of bridges with it htransmitter successively andcyclically to each of said ing diodes as described previously in twevlvesteps as folaerials n Said y lows: 2. A radio beacon as in claim 1, inwhich said means A4 A5 A6 A1 A8 A0 A; As A1 d A: A4

Repeated at One carrier phase As As An As A1 A0 A5 A4 A; A: 0 A7 30 c./

A7 As A5 A4 A4 A5 As A7 As A9 A!) A5 A0 s A7 Au A5 A4 A4 As A0 A1 s AsOpposite carrier Repeated at phase A5 A4 A4 A5 A5 A7 As Ar A; A A7 A0 30c./s.

A0 A1 As At n 5 A1 At A5 A4 A4 As It will be seen that each of thecycles involves coupling for successively and cylically coupling saidoutputs of said the end aerials A and A twice. transmitter to said arrayof aerials produces simultation One more embodiment will now bedescribed which of a pair of sources moving in opposite directions.produces exactly the same radiation field as the last em- 3. A radiobeacon as claimed in claim 2 further includbodiment, i.e. that shown inFIG. 6 at 90 e./s. This eming an aerial Within said array, and means toenergize said bodiment differs from the last only in that the l2-stepaerial with radio frequency energy in quadrature phase re- 30 c./ s.coupling cycle for the middle aerials A to A is lationship with theradio frequency energy energizing said simplified to one of 90 c./s.having only four steps, viz: simulated pair of aerials.

4. A radio beacon as claimed in claim 3 wherein said A4 A5 A5 A4 arrayof aerials is a horizontal linear array and said aerial one carrierphase A7 A5 A5 A1 Repeated at 90 MS. energized with the quadrature radiofrequency energy is at the mid-point of sald linear array. A8 AB 5. Aradio beacon as claimed in claim 4 wherein the cyclic successivecoupling of the aerials of the array to the Opposite carrier phase Ae A7A1 A6 Repeated at 90 c./s. trarismitter such that simgle harmonic motionof the oppositely moving sources 18 simulated. A8 A8 A9 6. A radiobeacon as claimed in claim 4 wherein a lincar saw-tooth movement of thesources is simulated. This simplification can be made because, as willbe seen 7. A radio beacon as claimed in claim 6 wherein opby inspectionof the above 30 c./s. sequences for the last posite ly phased movementsof a second pair of aerials embodiment, cyclic recurrence actuallyoccurs at 90 c./s. is simulated, the simulated oscillatory movements ofthe Except for the first embodiment, all of the others infirst andsecond pairs of aerials differing in phase. volve linear symmetricalsaw-tooth aerial coupling, which 8. A radio beacon as claimed in claim 7wherein the has the advantages over simple harmonic coupling thatoscillatory movements of the pairs of radiating sources the aerials arecoupled at equal intervals and, that the differinphase by 60 degrees.sidebands separate themselves considerably in azimuth A radio beaCOIl asm d in C aim 8 wherein Oparound the beacon. This latter fact is veryimportant in positely phased movements of a third pair and a fourthpractice. pair of aerials along the line is simulated, the phasedifference between the simulated oscillatory movements of the pairs ofaerials being 30 degrees.

10. A radio beacon as claimed in claim 7 wherein the phase differencesbetween the simulated oscillatory movements of the pairs of aerials are0, 36, 60 and 96 degrees, respectively.

11. A radio beacon as claimed in claim 4 wherein the radio frequencyenergy energizing the aerial at the midpoint of the linear array isamplitude modulated.

12. A radio beacon as claimed in claim 11 wherein the cyclic successivecoupling of the spaced aerials to the transmitter is at a firstfrequency and at a second frequency, and the radio frequency energyenergizing the aerial at the mid-point of the linear array is amplitudemodulated by signals at the first and second frequencies, the phasing ofthe amplitude modulations being such that on one side of the horizontalpenpendicular bisector of the linear array the amplitude modulations ofthe combined radiated fields at the first and second frequencies of thearray tend to add and to subtract, respectively, while on the other sideof the said bisector the amplitude modulations at the first and secondfrequencies tend to subtract and add, respectively.

13. A radio beacon as claimed in claim 4 wherein each of the end aerialsof said horizontal linear array is coupled to the transmitter for twoconsecutive coupling steps.

14. A radio beacon as claimed in claim 13 wherein twelve aerials areequally spaced on a straight line of length 41 at the transmitterfrequency and the oscillatory movements of four pairs of aerials aresimulated, and the phase difference between the simulated oscillatorymovements of the pairs of aerials being 30 degrees.

15. A radio beacon as claimed in claim 4 including means to simulate bysuccessive coupling of some of the spaced aerials to the transmitteroppositely phased oscillatory movements along the straight line of asecond pair of aerials energized with radio frequency energy in oppositephase to each other, the frequency of the oscillatory movements of thesecond pair of aerials being a sub-harmonic of the frequency of theoscillatory movement of the first pair of aerials.

16. A radio beacon as claimed in claim 1 wherein the spaced aerials areeach coupled to the transmitter by means of two switching diodes each atopposite corners of a bridge circuit having three arms of electricallength M4 at the transmitter frequency and a fourth arm of electricallength 3M4.

17. A radio beacon as claimed in claim 16 wherein the spaced aerials areeach coupled to the transmitter by means of two further switching diodeseach at opposite corners of a further bridge circuit having three armsof electrical length M4 at the transmitter frequency and a fourth arm ofelectrical length 3M 4.

References Cited by the Examiner UNITED STATES PATENTS 2,411,518 11/1946Busignies 343-106 2,521,702 9/1950 Eanp et a1 343-108 3,094,697 6/1963Kramar et a1. 343-106 3,115,634 12/1963 Karpeles 343-108 3,130,4074/1964 Kramar 343-106 CHESTER L. JUSTUS, Primary Examiner.

H. C. WAMSLEY, Assistant Examiner.

1. A RADIO BEACON FOR PRODUCING AN AMPLITUDE MODULATED FIELD, COMPRISINGAN ARRAY OF SPACED AERIALS, A TRANSMITTER HAVING FIRST AND SECONDOUTPUTS OF OPPOSITE PHASE, MEANS FOR COUPLING SAID FIRST OUTPUT OF SAIDTRANSMITTER SUCCESSIVELY AND CYCLICALLY TO EACH OF SAID AERIALS IN SAIDARRAY, AND MEANS FOR COUPLING SAID SECOND OUTPUT OF SAID TRANSMITTERSUCCESSIVELY AND CYCLICALLY TO EACH OF SAID AERIALS IN SAID ARRAY.